Nerve cells are cells that constitute the nervous system, which can be broadly divided into the central nervous system and the peripheral nervous system. Nerve cells are prone to be damaged by external factors such as a cerebrovascular injury such as a stroke, a brain infarction, etc.; or infernal factors such as an accumulation of abnormal proteins, oxidant stress, inflammation, etc., while their regenerative capacity is low. Thus, once nerve cells are damaged, those damaged nerve cells cause a marked reduction in the QOL of a patient. Neurodegenerative disorders that involve degeneration and loss of nerves of the central nervous system, include neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, multiple sclerosis, etc. as well as degenerative diseases of the optic nerve such as glaucoma, etc.; and neurosensory degenerative diseases such as neural hearing loss, etc.
With developments in neuro science, various neuroprotective factors have been discovered, and they are expected to developed as a drag for preventing or treating nerve damage. While agents that reduce free radicals or excitatory amino acids and agents that can protect and/or repair nerve cells (e.g., immunophilin ligands such as neurotrophic factors and immunosuppressants) have been found to have a neuroprotective action, it has also been found that biological proteins, such as pituitary adenylate cyclase-activating polypeptide (PACAP), CD44 and human brain carboxypeptidase B (HBCPB), have a neuroprotective action. (PTL 1 and 2)
Human tears contain numerous proteins such as lysozymes, lactoferrins, etc., which serve to maintain the homeostasis of the corneal surface. Moreover, tears also contain degradants of proteins, and it is expected that such peptide fragments have various actions. Proline-rich protein 4 (PRP4) known to be present in tears is a protein having a fall amino acid length of 134 residues. Since it has an antibacterial action, it is suggested that it acts as a protective mechanism against pathogens in mucous tissues (NPL 1).